Both are very comfortable depending. Your ears would rest on the AD900's hard drivers making it a bit annoying but the ad900 has slightly less clamping force than the dt880. The dt880 has a tiny clamping force that may be only present for long term use (many hours on your head), but again it is little and most people find them very comfortable. So they are about the same in comfort depending on what negative you are willing to deal with.

The dt880 is brighter than the ad900 (around 2-3khz) and has a stronger mid-high treble (the ad900 is on the bright side as well). The dt880 has more bass, and it doesn't bleed into the rest of the spectrum (it's nice). Some consider the ad900 to be slightly bass shy where they say the dt880 has the correct amount. The ad900 has a larger soundstage even though the dt880 has a decent size soundstage too.

So you can have something with more bass, more brighter, with good comfort and good soundstage or

you can have something a little more neutral (still bright but less bright and slightly on the bass shy of things) with a larger soundstage and good comfort.

Both are easy to drive and can be driven with portable amps (not essential though).

Some people can't stand their ears on the hard drivers of the ad900 yet some people can't stand bright headphones (more bass does help a little though).

Brightness is something you can get use too, plus burn it settles it down anyway. I had ultrasone headphones (bright) for many years. I use to hate them but I'm used to them now. Maybe it was their burn it or my brain burn it or both.